Bitumen is today being commercially extracted from tar sand using a recovery process commonly known as the hot water process.
In general, this process involves: mixing the mined tar sand with hot water, steam and sodium hydroxide in a tumbler; diluting the produced slurry with additional hot water; retaining the diluted slurry under quiescent conditions in an open-topped primary separation vessel having an outlet in its conical base, whereby aerated bitumen rises to form a primary froth, which is collected, and solids settle and are removed through the outlet; withdrawing a dragstream from the middle of the vessel, said dragstream containing non-rising bitumen and clay particles; and subjecting said dragstream to induced air flotation to recover a secondary froth.
The tar sand is a complex material. More particularly, it comprises: sand-size solid grains; connate water sheathing the grains; fine clay-like solids (-325 mesh) which appear appear to be concentrated in the water; and bitumen filling the interstices between the water-sheathed sand grains.
The concentrations of these various components, which make up the tar sand, vary throughout the deposit and hence in the feed led to the extraction plant. These variations in concentration have a marked effect on the efficiency of the recovery process.
The component whose concentration variations can most deleteriously affect the hot water process is the fine solids (hereinafter termed "fines"). The solids component of a low fines tar sand may contain in the order of 5% by weight fines, while the solids component of a high fines tar sand may contain in the order of 20% by weight fines.
Operators of the process can react to the presence of higher levels of fines in the feed by increasing the sodium hydroxide and water additions to the process; these increases will reduce the deleterious effects of the high fines on the efficiency of the bitumen recovery process.
To date, the practice used for monitoring the tar sand composition has involved sampling the feed and subjecting the samples to laboratory analysis. However, this is a time-consuming process and thus the implementation of changes in water and sodium hydroxide addition is late, with the result that the hot water process is rarely operated at optimum conditions.
There has thus existed a long-standing need for an on-line analysing means which would monitor and indicate tar sand feed grade trends accurately and quickly.
It needs to be noted that there is an inverse proportional relationship between bitumen and fines concentrations in tar sand. Therefore the development of an accurate indicator of bitumen content would provide the industry with a means for monitoring fines content.